Who's the guy to the left?

Judging by the wings and the Noxian lineup, I'm pretty sure it's Swain in bird form, with his back turned.

Its Morgana

Not Morgana, it is indeed Swain. The top outlines of Raven!Swain's wings have a golden outline (although in the picture, this may be attributed to the sun in the background). Morgana's wings are more broken up, Swain's wings are held together far more. Morgana does not wear a solid, body-covering coat, Swain does (although this may also be attributed to the picture's shading due to the sun). Not to mention the Noxian theme going on here with Cassiopeia and Katarina.

Look at those ears. Swain doesn't have ears in Raven form and the character is standing in Morgana's Tormented Soil. Also that posture is rather humanoid instead of bird-like. I think the theme here is not Noxian but more villainous females except for Riven and her unwavering warrior spirit.

Perhaps a more definite proof that it isn't swain:
Boobs present :P

Also, tag mentioned that there is a buckler present. I'm pretty sure that it's just part of the glove and more of a gauntlet rather than a shield

Im going to have to go with morganna on this one.. while swain does wear robes that really does look like morgana's dress and her tormented soil. Ill be changing it back. But that really doesnt looke like swain also those are morgana's angular wings

I am a bit more convinced that there could be no Noxian theme. The image screams it except for Morgana, if that is indeed Morgana. I'll leave the morgana tag there anyway, since we're supposed to tag what we see.

Although not Noxian by birth, Morgana does reside in Noxus.

As for buckler, the Redeemed splash art does have a buckler-shaped piece which is depicted even larger than the one here.

It may not be Noxus behind Riven, but "complete monsters".

Katarina is a monster by trade.
Cassiopeia is a monster by form.
Morgana is a monster by nature.

Riven left Noxus to try and avoid becoming a monster herself. Perhaps that's the theme, here.